In recent years, progress in an electronics field has successively led to research and development of new electronics materials. Such new material includes a new nanocarbon material such as a carbon nanotube and graphene. These materials are investigated on characteristics thereof, and development and applied research have made progress as a new electrically conductive material having a high electrical conductivity in place of a conventional metallic material.
Meanwhile, an image display device typified by a liquid crystal display (LCD), a plasma display panel (PDP), or an electroluminescence (EL) device has been applied to not only a television or a computer, but also units in various fields, such as various kinds of mobile units that have been widely used in recent years, and has made remarkable progress. In recent years, an energy shift to non-fossil energy has been required in view of a global environment, and therefore widespread use of a solar cell and an improvement in the function thereof are also required.
Such display device or solar cell uses a transparent electrically conductive film. The transparent electrically conductive film generally uses a metallic material such as ITO (indium tin oxide), and is formed by forming a film of the metallic material on a glass substrate by a vapor phase process such as a vacuum deposition process and a sputtering process. However, in the vapor phase process, control of conditions in a producing step is difficult, a large amount of cost is required for a production unit or the like, and enlargement of a film-forming area is also difficult.
Further, in electrical equipment such as a cellular phone and mobile equipment, substrate materials thereof has been shifted from glass to a plastic, for meeting a strong demand for reducing the weight and improving the flexibility. If a plastic substrate is used, weight of a display unit can also be reduced to a half or less of weight in a case of a glass substrate, and strength and impact resistance can also be significantly improved. However, the plastic substrate has insufficient heat resistance for use in a conventional vapor phase process which requires high heat resistance for the substrate. Moreover, use of the plastic substrate has other problems such as a decrease in adhesion of a formed coating film to the substrate, resulting in being easily detached.
In order to solve the problem, if an electrically conductive material containing the nanocarbon material such as the carbon nanotube can be prepared on a plastic-based material, film formation by a coating method such as spin coating can be made. In this film forming, a high temperature or vacuum conditions are not essentially required, and thus a production step is simple, and production cost can be suppressed. Further, a material such as the plastic can also be utilized as the substrate, and therefore can respond to achievement of light-weight or flexible equipment, and also allows a significant improvement in strength and impact resistance. Furthermore, the electrically conductive material is also suitable for production of a large area film. In view of such advantages, development and practical use of a new electrically conductive material using the nanocarbon material are expected.
Meanwhile, upon utilization of the carbon nanotube (hereinafter, also abbreviated as CNT) to the electrically conductive material, dispersibility of the carbon nanotube into a liquid is considered to be a problem to be solved. For example, when a thin film is formed using a CNT, the CNT is dispersed into a medium such as water and an organic solvent. However, the CNT is easy to aggregate, and not easy to be dispersed thereinto. In order to solve the problem, a dispersing agent is added to a dispersion liquid for improving dispersibility of the CNT, and the dispersion liquid is coated to form a film (for example, see Non-patent literature 1 or 2). As specific methods for improving dispersibility of a CNT using a dispersing agent, a method for dispersing the CNT into an aqueous solution containing a surfactant such as sodium dodecyl sulfonate (for example, see Patent literature 1), a method using a cationic surfactant, an anionic surfactant, or a nonionic surfactant (see Patent literature 2), a method using an anionic dispersant (see Patent literature 3), and so forth are proposed.
However, these dispersion methods are limited to dispersion into an aqueous medium, and may occasionally cause a defect in the CNT during dispersion. Moreover, if an electrically non-conductive dispersing agent adheres onto a CNT surface, the electrical conductivity and semiconductor characteristics proper to the CNT are adversely affected, and thereby decreasing performance (see Non-patent literature 3).
In order to solve the problem, a method in which a conjugated polymer, so-called electrically conductive polymer, allowing transfer of electric charges between the polymer and the CNT is used to disperse the CNT into a solution of such a polymer and to improve dispersibility (for example, see Patent literatures 4 to 9 or Non-patent literature 4) is proposed. However, even by such a method, sufficient development of high electrical conductivity to the CNT is difficult.